Session Availability on IdentifI Wireless Controller C35

I have few question about Session Availability on IdentifI Wireless Controller C35.

I have three controller on different location and each Controller have own APs. If I need to use C35-No.1 is Primary for APs-No. 1 to 25 and Backup for APs-No. 26 to 50, C35-No.2 is Primary for APs-No. 26 to 50 and Backup for APs-No. 51 to 75, C35-No.3 is Primary for APs-No. 51 to 75 and Backup for APs-No. 1 to 25. (can see in picture).